The art of inkjet printing technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ inkjet technology for producing hard copy printed output. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Volume 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Inkjet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
A thermal inkjet printer for inkjet printing typically includes one or more translationally reciprocating print cartridges in which small drops of ink are formed and ejected towards a medium upon which it is desired to place alphanumeric characters, graphics, or images. Such cartridges include a printhead having an orifice member or plate that has a plurality of small nozzles through which the ink drops are ejected. Adjacent to the nozzles are ink firing chambers, in which ink resides prior to ejection through the nozzle. Ink is supplied to the ink-firing chambers through ink channels that are in fluid communication with an ink supply, which may be contained in a reservoir portion of the print cartridge or in a separate ink container spaced apart from the printhead.
Ejection of an ink drop through a nozzle employed in a thermal inkjet printer is accomplished by quickly heating a volume of ink within the adjacent ink firing chamber by applying an energizing electrical pulse to a heater resistor positioned in the ink firing chamber. The electrical pulse induces a temperature rise in the heater resistor, which heat energy is transferred to the ink to produce an ink vapor bubble. The rapid expansion of the ink vapor bubble forces ink through the nozzle. Once ink is ejected, the ink-firing chamber is refilled with ink from the ink channel and ink supply. The energy required to eject a drop of a given volume is referred to as turn on energy. The turn-on energy is a sufficient amount of energy to form a vapor bubble having sufficient size to eject a predetermined amount of ink through the printhead nozzle.
Significant effort has been expended in improving print quality. Since the image output of an inkjet printer is formed of individual ink drops, the image quality and contrasts, as well as variations in image hue and lightness, are dependent on ink drop volume and ink drop distribution on the printed medium. It is known that drop volumes vary with the printhead substrate temperature because the properties that control it vary with temperature: the viscosity of the ink itself and the amount of ink vaporized by a heater resistor when driven by a given electrical printing pulse. One method of controlling drop volume is to vary the electrical pulse width supplied to the heater resistor (see U.S. Pat. No. 5, 726,690). However, inkjet ink is chemically reactive and prolonging of the exposure of the heater resistor and its electrical connections to the ink may result in a chemical attack upon the heater resistor and a deterioration in the long term performance of the heater resistor. Another method of controlling drop volume is to construct a protective layer having a thickness gradient over the heater resistor (see U.S. Pat. No. 4,339,762; see also U.S. patent application Ser. No. 09/302,178, entitled Variable Drop Mass Inkjet Drop Generator, filed Apr., 29, 1999 and assigned to the assignee of the present invention and hereby incorporated by reference herein). However, varying the thickness of the protective layer is subject to the tolerances of the semiconductor manufacturing process and to the tolerances in the heat conduction gradients of the protective materials. Therefore, a need exists for utilizing the printhead substrate temperature in order to intentionally produce ink drops of varying volumes, facilitating additional control and an expanded range of intended variations in hue and lightness of color in images, without the need to vary the electrical pulse width or to vary the thickness of the protective layer.